BUG allow outlier_label=0 in RadiusNeighborClassifier

Fixes #1602.
BUG in RadiusNeighborClassifier outlier handling

Ruggedized the test, which wasn't doing much previously, and added a test
for c0d4015d201b68e5da6afe4c7fb8b0e6819f038c.

Also replaced flatten (which copies) with ravel and improved the error
message for outliers in the face of outlier_label=None.